1. Field of the Invention
The present invention relates to a mold for injection molding of a thermoplastic resin, particularly to a mold used for fabricating thermoplastic resin articles, which makes possible the accurate transfer of the gloss or fineness of the cavity surface to the articles without any sink marks thereon even at a low pressure.
2. Description of the Related Art
The known molds used for fabricating thermoplastic resin articles are made of metals such as conventional steel, nickel, nickel alloys, aluminum, aluminum alloys, copper, copper alloys, etc. A molten resin is introduced into a cavity under pressure to be molded to a predetermined shape and size. In this molding technique, generally a more precise article with good appearance can be obtained compared with other molding techniques such as blow molding, etc.
However, in the case where the article is fabricated by injection molding, using such a conventional mold, the pattern on the cavity surface is difficult to transfer to the article without blurring or losing its original fineness and gloss. If such blurring occurs in the pattern transferred to the article, the appearance of the article is spoiled, and its commercial value is lost. In addition, weld lines and flow marks are likely to remain on the surface of the article, as well as sink marks are likely to remain near an end part or on thick wall portions of the article.
In order to solve the above-mentioned problems, generally, the temperature of the mold or the injection pressure is increased. However, there is a limit to the improvement of the transfer accuracy even though the temperature of the mold is increased. In addition, the increase in temperature of the mold leads to a prolonged cycle time of molding. Thus, satisfactory results cannot be obtained. As for the increase in injection pressure, there is also a limit to the improvement of the transfer accuracy. The increase in injection pressure causes warpage due to residual strain, and causes much flashing, etc., resulting in degraded quality of articles. Moreover, the increase in injection pressure needs the enlargement of a molding machine, and the strengthening of the mold. Accordingly, this makes it difficult to economically fabricate satisfactory articles.
The reasons for the limit to the quality improvement of the article using a conventional mold are as follows:
The mold for injection molding is made of metals which generally have good thermal conductivity. When a molten resin is introduced and comes into contact with the cavity surface, the molten resin gives off heat to the cavity surface with rapid cooling; consequently a solid layer is immediately formed on the surface of the molten resin. More precisely, during the stage of mold filling, or flowing of the resin into the cavity, the internal pressure of the molten resin is relatively low, and the rapid solidification takes place on the very surface of molten resin prior to the completion of filling. These prevent the molten resin from coming into full contact with every intricate part of the pattern on the cavity surface. That is, the mold surface pattern cannot be transferred to the articles without losing its original fineness, thus resulting in poor appearance, even when a high pressure is given at the final stage of filling.
In recent years, there has been a demand to produce thin and large sized articles with good surface quality at a low cost.
In order to fabricate thin and large articles mentioned above, higher molding pressure has generally been used. The application of a higher pressure using a conventional mold, however, cannot meet this demand, because of giving articles unimproved appearance and secondary problems such as warpage and much flashing. In other words, there is a limit to produce the than and large articles with good appearance by increasing the pressure.
Much effort has been made, therefore, to develop new molding technology for obtaining good articles at a relatively low pressure. There are a number of known studies and technologies related to this subject as follows:
1) Low expansion injection molding for preventing sink mark.
2) Hettinger process: low pressure injection foaming technique.
3) SINPRES process: injection molding with gas introduction.
4) Injection compression molding.
5) Injection molding with an insulated mold.
6) Injection molding with a cavity surface treated mold: coating of an insulating and wettability-giving thin film onto a cavity surface.
In the above-mentioned processes 1) to 4), sink marks are substantially prevented from being formed. However, the unsatisfactory transfer of the pattern from the cavity surface to the article cannot be improved due to the fast solidification of the surface layer of the molten resin during filling, and phenomena of flow marks and weld lines also remain to be unimproved. Thus, articles with good surface quality cannot be obtained at a low molding pressure by these processes.
In the process 5), the solidification of the surface layer of the molten resin is delayed, and in the process 6), the adhesion between the injected molten resin and the cavity surface is improved due to the coating film therebetween. Thus, the transfer of the pattern from the cavity surface to articles is improved. However, under the condition of a low pressure, sink marks cannot be prevented from being formed near the end portions of the article against gate and/or on the thick wall portions of the articles, requiring high pressure molding for eliminating them.
As just described, articles with good quality cannot be fabricated at a low pressure even with the use of the above-mentioned processes.